alternatives: x86 rename and change parameters on ARM

alternatives: x86 rename and change parameters on ARM

On x86 we squash 'apply_alternatives' in to
'alternative_instructions' (who was its sole user)
and 'apply_alternatives_nocheck' to 'apply_alternatives'.

On ARM we change the parameters for 'apply_alternatives'
to be of 'const struct alt_instr *' instead of void pointer and
size length.

We also add 'const' and make the arguments be on the
proper offset.

Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com> [x86 bits]
Reviewed-by: Julien Grall <julien.grall@arm.com> [ARM bits]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>